Architecture variant: more responsible Redirect Proxy

[rfc2822]

Architecture variant where we let the Redirect Proxy act as RFC 8030 Push Service:

• simplified architecture, especially for on WebDAV server side (no requirement to support proprietary protocols here)
• more complicated Redirect Proxy, because it now also has to
  • manage the subscriptions / topics and
  • create a Web Push subscription for a topic if there's at least one client,
  • register the Web Push subscription to the WebDAV server

[rfc2822]

Closing because this is not our current favorite architecture. Please see #21 for our current favorite (and reasoning) and comment there if you have feedback, which is very welcome.